fix: roll back rejected general settings save

Issue: #439
User-Visible: yes

src/serialized-write-queue.ts

@@ -9,3 +9,52 @@ export function enqueueSerializedWrite(
 ): Promise<void> {
   return previous.catch(() => undefined).then(write);
 }
+
+export interface OptimisticAttempt<T> {
+  previous: T;
+  previousFingerprint: string;
+  revision: number;
+  attempted: T;
+  attemptedFingerprint: string;
+}
+
+/** Capture the server-backed value before exposing a write candidate locally. */
+export function optimisticAttempt<T>(
+  previous: T,
+  attempted: T,
+  previousFingerprint: string,
+  revision: number,
+  fingerprint: (value: T) => string,
+): OptimisticAttempt<T> {
+  return {
+    previous: JSON.parse(JSON.stringify(previous)) as T,
+    previousFingerprint,
+    revision,
+    attempted,
+    attemptedFingerprint: fingerprint(attempted),
+  };
+}
+
+/**
+ * Roll back only the failed candidate represented by this attempt. A conflict
+ * reload or a newer edit owns a different revision/content and must win.
+ */
+export function rollbackOptimistic<T>(
+  host: {
+    _serverCfg: T | null;
+    _cfgRev: number;
+    _cfgContentFingerprint: string;
+    requestUpdate: () => unknown;
+  },
+  attempt: OptimisticAttempt<T>,
+  fingerprint: (value: T) => string,
+): boolean {
+  const current = host._serverCfg;
+  if (!current || host._cfgRev !== attempt.revision
+      || (current !== attempt.attempted
+        && fingerprint(current) !== attempt.attemptedFingerprint)) return false;
+  host._serverCfg = attempt.previous;
+  host._cfgContentFingerprint = attempt.previousFingerprint;
+  host.requestUpdate();
+  return true;
+}